Field of the Invention
The present invention relates to a novel electrochromic organic compound and an electrochromic element containing the organic compound.
Description of the Related Art
Various electrochromic (hereinafter also referred to as “EC”) materials have been reported. The optical absorption characteristics (coloration or light transmittance) of electrochromic materials change through an electrochemical oxidation-reduction reaction. Metal oxides, such as WO3, are known as inorganic EC materials. However, films of metal oxides are principally formed by vapor deposition, and it is difficult to form large metal oxide films.
Examples of known organic EC materials include electrically conductive polymers described in Japanese Patent Laid-Open No. 56-67881 and organic low molecular compounds, such as oligothiophenes described in David D. Graf and Kent R. Mann, Inorganic Chemistry, 1997, 36, 141. An electrically conductive polymer described in Japanese Patent Laid-Open No. 56-67881 can form an EC layer directly on an electrode by the electrolytic polymerization of a monomer.
Examples of known electrically conductive polymers that can form an EC layer include polythiophene, polyaniline, and polypyrrole. Electrochemical oxidation or reduction of these electrically conductive polymers results in changes in the π-conjugated chain length of the main chain, the electronic state of the highest occupied molecular orbital (HOMO), and the absorption wavelength.
These electrically conductive polymers have a long π-conjugated system and are colored in their neutral state because of absorption in the visible light region. The oxidation of the electrically conductive polymers shifts their absorption wavelengths toward the long wavelength side (infrared region side). A shift toward the infrared region side results in no absorption in the visible light region, and the EC element loses color.
An EC material of an oligothiophene compound described in David D. Graf and Kent R. Mann, Inorganic Chemistry, 1997, 36, 141 has a shorter π-conjugated chain length than electrically conductive polymers.
Thus, oligothiophene EC compounds in their neutral state often have an absorption peak in the ultraviolet region and look transparent to the human eye. An oxidation reaction converts an oligothiophene to a radical cation, which has absorption in the visible light region and is colored.
Organic EC materials are sometimes not transparent even in their colorless state. No organic EC material is known to have both high transparency in its colorless state and absorption in a visible long wavelength region in its colored state. Organic EC materials are often unstable under repeated oxidation and reduction.
An electrically conductive polymer described in Japanese Patent Laid-Open No. 56-67881 in its neutral state has an absorption band in the visible light region. Thus, a portion subjected to an insufficient electrochemical reaction remains colored even in its oxidized state and rarely has high transparency.
The stability of electrically conductive polymers is increased through intramolecular delocalization of a generally unstable radical cation. Nevertheless, because of their insufficient stability, electrically conductive polymers may deteriorate during repetitive oxidation-reduction reactions and consequently have poor performance.
Organic low-molecular oligothiophene EC compounds in their neutral state often have absorption in the ultraviolet region. With an increase in the number of thiophene rings constituting an oligothiophene EC compound and the conjugation length, the absorption edge of the oligothiophene EC compound extends to the visible range, and transparency in the colorless state decreases.
In fact, in an EC compound based on a thiophene trimer derivative described in David D. Graf and Kent R. Mann, Inorganic Chemistry, 1997, 36, 141, a structure composed of three thiophene rings has a relatively long conjugation length.
Thus, while the absorption peak in an oxidized colored state is approximately 660 nm in the visible long wavelength region, the absorption edge in the neutral state extends to approximately 500 nm. Thus, the EC compound has visible absorption and insufficient transparency even in its colorless state.
When the number of thiophene rings is decreased to shorten the conjugation length in order to improve transparency in the neutral colorless state, it is known that the visible absorption wavelength in the oxidized colored state is also an absorption band in the range of approximately 400 to 600 nm.
Thus, it is difficult to achieve both high transparency in the colorless state and absorption in a long wavelength region, particularly absorption in a long wavelength range of 600 nm or more, in the colored state.
Furthermore, because of a shorter conjugation length of an oligothiophene compound than electrically conductive polymers, there is also a problem in radical cation stability in the oxidized state.